1. Field of Invention
This invention relates to a method of detecting a particular underground optical transmission line from the surface of the ground.
2. Prior Art
In modern telecommunication systems, optical cables are laid along the route of installation directly or housed in a duct or conduit which is lying underground.
Typical underground optical telecommunication networks comprise central stations, relay stations and terminal stations interconnected by means of a large number of optical transmission lines, which are often ramified, looped and star-connected.
There often arises cases where a particular underground optical transmission line needs to be branched, moved, tested, repaired and replaced.
Then, the particular underground optical transmission line has to be detected and dug out from the surface.
Since the routes of underground optical transmission lines are documented, it is normally not difficult to roughly trace the route of a particular transmission line from the ground surface and select a site for digging out the line. However, tremendous efforts may be required to precisely locate a particular transmission line at the work site simply because it is not visible from the surface. An old and conventional method to alleviate such efforts is to put a sign, or indicator, on the ground, telling that one or more than one optical transmission lines are buried there.
As more and more underground optical transmission lines are laid to form large networks, however, installation of such signs will become costly and, in some cases, may constitute obstacles for traffic on the ground.
Additionally, such indicators will eventually become damaged, degraded and inoperative as they are exposed to the atmosphere for a prolonged period of time.
In short, the use of signs does not provide an effective way for detecting underground optical transmission lines and is accompanied by a problem of necessity of digging the ground over a large area to take out a single transmission line. If an excavator machine is used to efficiently dig the ground over a large area, it can accidentally destroy transmission lines and/or other underground installations.
It should be noted here that an underground optical transmission line comprising an electric conductor or a metallic component can be precisely located by sending an electric signal into the conductor and detecting from the ground surface the electromagnetic wave generated by the conductor according to the electric signal.
As this technique is effective only for optical transmission lines comprising a conductor, the use of the technique will inevitably be very limited under the current circumstances where non-metallic optical transmission lines are more and more prevalent in optical telecommunication systems.